I found that the DS Camera Stream displays the last-built VisionPortal.
In quick testing, I simply changed the order of the Functions shown here with green arrows:
The webcam associated with the second-built portal is the one that displays in Camera Stream. This was repeatable through random changes and multiple cycles. The webcams were plugged directly into the USB 2.0 and USB 3.0 ports on the Control Hub.
This test OpMode is the one previously referred to you:
Actively switching or toggling the DS Camera Stream source is not currently offered in the FTC SDK. A simple solution would be a specialty OpMode to preview only the first-built portal in INIT.
Reminder, both previews are available simultaneously in the RC LiveView, before and after touching the START button, wirelessly via scrcpy.